Chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) are tumors of mature B cells and are closely related biologically and in clinical behavior. Both are currently incurable with chemotherapy. Because many patients are older, application of allogeneic transplantation and tolerability of aggressive chemotherapy is limited. Median survival for patients with CLL is 10 years and for MCL patients ranges between 5 and 6 years. Thus, there is a need to develop novel treatments, especially targeted agents and more tolerable immunotherapeutic approaches. The recognition that the B-cell receptor (BCR) repertoire expressed on CLL cells is highly skewed led to the hypothesis that antigen selection plays a role in disease pathogenesis. Frequently, these antigens appear to be auto-antigens leading to the concept that CLL is a malignancy of auto-reactive B-cells. Recent data suggest that the BCR repertoire in MCL is also skewed and by extension, that MCL similar to CLL is a disease of antigen-selected B cells. BCR signaling has emerged as the pivotal pathway in the pathogenesis of CLL. A major contribution from my group has been the first demonstration of active BCR signaling in CLL patients in vivo. Our findings support the importance of the BCR for disease progression and identify the pathway as a relevant target for therapeutic intervention. Furthermore, we showed that BCR signaling and the consequent activation of the NF-B pathway occurs primarily in the lymph node microenvironment rather than in the peripheral blood or bone marrow. Thus, in key aspects, the biology of CLL is shaped by its environment; an insight that changes our therapeutic approach, the design of correlative studies, and the development of model systems. We have shown that BCR signaling and activation of the NF-B pathway in CLL cells occurs primarily in the lymph node microenvironment. To expand on these observations, we conducted a clinical study using deuteriated water to label the proliferative fraction of the CLL clone in vivo (NCT01117142), which provided direct in vivo evidence for increased tumor proliferation in the lymph node. Furthermore, in collaboration with Dr. Chiorazzis group we could show that a distinct subpopulation of CLL cells in the lymph node contains the proliferative core of the disease. Having shown the importance of the lymph node microenvironment for CLL cells, we developed a preclinical model system that can reproduce crosstalk between tumor and host microenvironment. As there are currently no good cell line or mouse models of CLL, we adapted a recently described mouse xenograft model and validated that human CLL cells engrafting in the murine spleen proliferate and undergo activation of BCR and NF-B pathways, similar to what we have previously found in the human lymph node. We used this patient-derived xenograft model in conjunction with a transgenic murine CLL model to investigate the effects of kinase inhibitors and their combination on BCR signaling and anti-leukemic activity and showed that the combination of acalabrutinib (a second generation BTK inhibitor) and ACP-319 (a PI3K inhibitor) was superior to single-agent treatment in the murine model, warranting further investigation of this combination in clinical studies. Several small molecule inhibitors of BCR signaling and of PI3K have entered clinical development in CLL and other hematologic malignancies. The most promising clinical results have been achieved with ibrutinib (an irreversible inhibitor of BTK) and idelalisib (an inhibitor of PI3K). From January 2012 to January 2014 we accrued 86 patients into our single agent ibrutinib trial (NCT01500733). We recruited patients with an unmet clinical need, the elderly who are often unable to tolerate current aggressive standard therapies, or those with a deletion of chromosome band 17p13.1 (del 17p). The latter patients have a particularly poor outcome with chemotherapy and are in greatest need for novel approaches. We found that Ibrutinib induced responses in over 90% of patients irrespective of del 17p that were durable. At a median follow-up of 24 months none of the patients without del 17p has progressed. The estimated rate of progression for previously untreated patients with del 17p who received Ibrutinib as first-line therapy was 9%. The estimated rate of progression for relapsed or refractory patients with del 17p who received Ibrutinib as salvage therapy was 20%. These results are much better than what can be achieved with current standard of care chemotherapy regimens and suggest that Ibrutinib may become the preferred agent for these patients. Some side effects may limit ibrutinib treatment for a subgroup of patients, especially for those who develop atrial fibrillation. We are currently investigating a second generation BTK inhibitor in a clinical trial that seems to be equally efficacious but may have a different side effect profile. Because loss of function mutations in BTK causes a severe immune defect known as Brutons agammaglobulinemia, assessing the impact BTK inhibition on immune function is important. We found that immunoglobulin (Ig) G levels remained unchanged on treatment, while IgA increased. At the same time the frequency of infections decreased. Interestingly, patients with >50% improvement in IgA had fewer infections than patients with <50% improvement. We also conducted a pilot study using influenza vaccine and could show that patients on ibrutinib respond at least as well as untreated CLL patients to the vaccine, demonstrating that patients on ibrutinib can still mount a meaningful antibody response. Given that infections are a leading cause of death, this data have important preventive health implications and we will further investigate how immune function in our patients can be improved. We completed a detailed biologic analysis on the impact of Ibrutinib on the tumor cells in the blood and lymph nodes of patients with CLL treated on our clinical trial. We validated the on-target effect by demonstrating inhibition of BCR and NF-kappa B signaling and decreased tumor proliferation. Interestingly, the degree of inhibition of BCR signaling in the lymph node positively correlated with the degree of reduction in lymphadenopathy. We characterized the treatment-induced lymphocytosis seen in patients treated with ibrutinib and the effect of the drug on platelet aggregation and coagulation. More recently, we focused on mechanisms of drug resistance in CLL patients treated with ibrutinib. With median follow-up of 34 months, 15 of 84 evaluable patients (17.9%) progressed. Histologic transformation occurred in 5 patients (6.0%) and was limited to the first 15 months on ibrutinib. In contrast, progression due to CLL in 10 patients (11.9%) occurred later, diagnosed at a median 38 months on study. At progression, mutations in BTK (Cys481) and/or PLCG2 (within the autoinhibitory domain) were found in 9 patients (10.7%), in 8 of 10 patients with progressive CLL, and in 1 patient with prolymphocytic transformation. Applying high-sensitivity testing (detection limit 1 in 1000 cells) to stored samples, we detected mutations up to 15 months before manifestation of clinical progression (range, 2.9-15.4 months). In 5 patients (6.0%), multiple subclones carrying different mutations arose independently, leading to subclonal heterogeneity of resistant disease. Median survival from the time of progression was19.8 months, identifying ibrutinib resistance as a new high-risk feature of patients with unmet clinical needs. Ongoing efforts in the laboratory and clinic are investigating strategies to prevent and or overcome the development of ibrutinib resistance.